25 June 2013

The cheetah and the hare: the great muscle match-up

There is one fact that everyone knows about cheetahs.

They’re adorable.

Actually, the one fact that everyone knows about cheetahs is that they are the fastest land animal. A recent paper came up with new estimates for just how fast (26 meters per second!), but didn’t answer the question of exactly how it is these animals outperform every other beast on land. So let’s do a quick stretch before taking a run at the answer...

Ultimately, the ability to move depends on muscle. There are different types of muscles: some generate lots of power fast* but tire easily, while others are not as explosive* but won’t tire easily. A forthcoming paper by West and colleagues compares cheetah muscles to another species to see if there is something exceptional about the cheetah’s muscles that can help explain this lanky cat’s stunning sprints.

In this corner, we have the undisputed speed champion, Acinonyx jubatus.

And in this corner, we have the challenger:

A bunny.

Rabbits are quick, but they are not in the cheetah’s league for speed. West and colleagues tried to compare the power output of the muscles from these two mammals, predicting that the cheetah’s muscle power would easily outstrip the rabbit’s power.

The rabbit won.

It wasn’t just that the the cheetah’s muscles and the rabbit’s muscles had comparable power, which would have been unexpected enough. The rabbit had significantly more power on a straight-up muscle fibre to fibre comparison.

This may not be the last word on this subject. The cheetah sample posed some problems. The team got the sample when a captive cheetah died unexpectedly, so the authors had to preserve the cheetah’s muscle tissue instead of working with fresh tissue. Estimating muscle power from preserved tissue is not a straightforward “put it in the machine and read out the number” measurement. There are assumptions in estimating power, the types of fibre, and so on.

All of these mean that it is possible that the muscles of cheetah’s produce more power than a rabbit when the muscles are in an intact, living cheetah. The authors suggest cheetah muscle power might be half again what they measured here, due to factors like the temperature of the living animal.

Even so, it may be that the secret to the cheetah’s speed is not so much in the physiology of the muscles, but how those muscles are attached to the skeleton, how the skeleton is shaped, or other factors.

And maybe, just maybe, like pro sports announcers keep telling us in races, it all comes down to just how bad the cheetah wants it.

* Dear physicists: Yes, I know I may be using this imprecisely. I will accept any rebukes in the comments.